1. Field of the Invention
The present invention relates to an electronic circuit for use in an analog-to-digital (A/D) converter, and more particularly, to a current interpolation circuit for interpolating requisite reference voltages.
2. Description of the Prior Art
A conventional flash type A/D converter is one of the fastest structure which directly utilizes 2.sup.n- 1 (n is the number of bit resolution) comparators to compare the input analog voltage with 2.sup.n- 1 reference voltages and then generate n bits output digital code. The disadvantage of this structure is its large number of comparators needed, thus increasing power consumption and chip area.
A conventional folding type A/D converter is used to reduce the number of comparators. In this configuration, because all 2.sup.n- 1 reference voltages are folded into 2.sup.m reference voltages (m&lt;n) by a number of folding amplifiers, only 2.sup.m comparators are needed in this structure. In order to reduce the number of folding amplifiers required, the interpolation technique is commonly used in the folding structure. Many interpolation circuits are used to interpolate the output voltage by means of a number of series-connected impedance elements. However, this technique is not suitable for high speed A/D converter due to its need of converting the output current from the transconductance stage to the desirable voltage, which will disadvantageously produce an extra pole.
Current output from the folding amplifier is recently applied in several papers, for instance, IEEE JSSC, Jul. 1996, pp. 938-944 and IEEE JSSC, Sep. 1996, pp. 1248-1257, as shown in FIG. 2 and FIG. 3. Unfortunately, the conventional interpolation circuits mentioned above require a great number of transistors to its implementation, which blocks the improvement for the modern integrated circuits. For the foregoing introduction, there is a need for an interpolation circuit used in an analog-to-digital (A/D) converter to substantially reduce transistor count and its power consumption.